The existing door opening systems usually consist of two sections: indoor intercom and outdoor intercom, which are connected with or without wires. In these systems, after ringing doorbell by a visitor, outdoor intercom will be linked to indoor intercom and the inhabitant can have Audio or Audio/Video communication with the visitor, depending on the audio/video features available in the system and can open the door by indoor intercom, if necessary. In the absence of inhabitant, obviously the system lost its capacities. In this case, the visitor usually leaves a note or inquiries about the location and the returning time of the inhabitant from adjacent doors and finally leaves the place without any results. In these systems, the inhabitant will only be informed on visitors when he/she is physically present in the place, Therefore in his/her absence can't be informed on people going to visit him/her.
On the other hands, the necessary of access to indoor intercom makes it difficult for disable or elderly people to use, even while they present at the home.
Other major disadvantage of these systems is the shortage of security level of the estate, because when a thief rings the doorbell and doesn't receive a reply, it ensures him/her that nobody presents and the estate is empty.